The present invention relates to a vulcanizable rubber composition of the type comprising a cross-linkable unsaturated chain polymer base, which has a preferredxe2x80x94but not exclusivexe2x80x94use in the production of treads for vehicle tires.
More particularly, the present invention relates to a vulcanizable rubber composition comprising a cross-linkable unsaturated chain polymer base including natural rubber and at least a copolymer obtainable by polymerizing at least a conjugated diolefin with at least a vinyl aromatic hydrocarbon, as well as a carbon black-based reinforcing filler.
The invention also refers to a tread and to a tire obtainable from said composition.
As is known, in the field of vehicle tire manufacture one of the more difficult objects to be reached has always been that of reducing the rolling resistance of the tire, achieving at the same time a good wear resistance and a satisfactory skid resistance on wet road.
It is also known that the difficulties for achieving the aforementioned object arise essentially from the fact that the tire rolling resistance on the one hand and the wear resistance and wet skid resistance on the other hand, are affected in an entirely opposite manner by the amount of reinforcing fillerxe2x80x94substantially carbon blackxe2x80x94employed in the composition used to manufacture the tire tread.
In order to reduce the rolling resistance of the tire, in fact, it would be desirable to reduce the amount of the reinforcing filler used in the composition (for instance, below 70 parts every 100 parts by weight of the composition polymer base): in so doing, however, a falling off to unacceptable values of both wet skid resistance and wear resistance of the tire has been observed.
In order to overcome somehow this limitation of the compositions comprising carbon black-based fillers, the prior art has suggested to partly or fully replace the latter by so-called xe2x80x9cwhitexe2x80x9d fillers, in particular silica, as is described for instance in European Patent application EP 0 501 227.
Even though silica-based reinforcing fillers have allowed to reduce the tire rolling resistance without significantly affecting wear resistance and wet skid resistance, their use is not devoid of drawbacks.
In fact, these reinforcing fillers, besides having a cost much higher than the cost of carbon black, have per se a poor affinity with the polymer base of the compositions employed in the manufacture of tires andxe2x80x94as suchxe2x80x94require the use of suitable coupling agents that may chemically bind silica to the polymer matrix.
However, the need of using such coupling agents limits the maximum temperature attainable during the mixing and mechanical working operations of the composition, not to trigger off an irreversible thermal degradation of the coupling agent.
But the respect of the aforementioned temperature constraint involves a marked reduction in the very mechanical mixing action that is of the essential for an optimum dispersion of silica throughout the polymer matrix.
The ensuing insufficient dispersion of silica in the composition causes in its turn several drawbacks, basically related to the extreme variability and non homogeneity of the physico-mechanical characteristics of the composition from zone to zone of the same.
In the different attempts made to manufacture a tread starting from compositions including silica as main reinforcing filler, a remarkable drawing difficulty and a size variability of the tread obtained from the so-produced compositions have been observed.
To all these drawbacks another, non negligible one must be added, namely the reduced production capacity of the drawing apparatuses used for tread manufacture.
All this reduces the hourly production capacity of the plants, which causes an increase in production costs.
Lastly, the electric conductivity of tires including silica-based reinforcing fillers, is drastically reduced, which involves an accumulation of electrostatic charges that, in some cases, may disturb the electronic apparatuses which are aboard the vehicle or even cause sudden electric discharges of dangerous intensity.
The technical problem underlying the present invention is therefore that of providing a rubber composition including carbon black as main reinforcing filler, which allows to reduce the rolling resistance without affecting the wear resistance and wet skid resistance of the tire.
According to the invention, the above problem is solved by a vulcanizable rubber composition comprising a cross-linkable unsaturated chain polymer base including natural rubber and at least a copolymer obtainable by polymerizing at least a conjugated diolefin with at least a vinyl aromatic hydrocarbon, said composition further comprising at least a carbon black-based reinforcing filler, characterized in that:
a) said at least one copolymer is obtainable by polymerzing in 1,2 form with the vinyl aromatic hydrocarbon at least 50% by weight of the conjugated diolefin, so as to obtain in the copolymer a quantity of from 30% to 70% by weight to the total weight thereof of an olefin fraction having a 1,2 structure, and
b) said carbon black has a DBP absorption value measured according to ISO 4656-1 equal to at least 110 ml/100 g, a reduction in the DBP absorption value as measured after compression according to ISO 6894 equal to at least 25 ml/100 g and a surface area as measured by means of CTAB absorption according to ISO 6810 not greater than 120 m2/g.
In the following description and in the appended claims, the term: 1,2 polymerization, is intended to indicate a particular stereospecific polymerization method between the conjugated diolefin and the vinyl aromatic hydrocarbon by means of which the formation of a copolymer is obtained in which the olefin fraction comprises a prefixed quantity of side-chain vinyl groups xe2x80x94CHxe2x95x90CH2 bound to the polymer chain.
Methods for the 1,2 polymerization of conjugated olefins are well known in the art and are described, for instance, in U.S. Pat. Nos. 3,451,988 and 4,264,753.
According to the present invention, it has been surprisingly observed that the rolling resistance of a tread may be advantageously reduced, even when carbon black-based reinforcing fillers are used in the starting composition, without affecting neither the wear resistance nor the wet skid resistance of the tire.
This may be achieved by using in the composition a particular polymer base including a so-called xe2x80x9chigh vinylxe2x80x9d copolymer (i.e. a copolymer wherein at least 50% by weight of the conjugated diolefin polymerizes in 1,2 form with the vinyl aromatic hydrocarbon) and a carbon black having particular characteristics of structure and surface area.
This result is all the more surprising if one considers that high vinyl polymers fall among those that the known art suggests to use in association with silica-based fillers, i.e. those fillers that are delegated to replace carbon black, but not with carbon black per se.
See on the matter, for instance the article xe2x80x9cComparison of the Elastomer Filler Interaction between Chemically Modified and Emulsion SBR with Carbon Black and Silica Fillersxe2x80x9d, by Ayala et al., delivered at the meeting of the American Chemical Society, Rubber Division, held in Cleveland, Ohio, U.S.A., from Oct. 17 to 20, 1995.
From the experimental tests mentioned in said article, in fact, it may be inferred that while a high content of vinyl groups in the polymer base facilitates on the one hand the incorporation of silica in the composition, because of the greater affinity with the latter, it has, on the other hand, a deleterious effect on the rolling resistance of the compositions including carbon black, which drops to unacceptable values (see, for instance, Table VII).
According to the invention, the conjugated diolefin/vinyl aromatic hydrocarbon copolymer may be obtained by polymerizing in 1,2 form at least 50% by weight of the conjugated diolefin to the total weight of conjugated diolefin used in the polymerization.
Preferably, the conjugated diolefin/vinyl aromatic hydrocarbon copolymer of the invention is obtained by polymerizing in 1,2 form a quantity of conjugated diolefin of from 60 to 70% by weight to the total weight of the diolefin used in the polymerization.
Preferably, furthermore, the conjugated diolefin/vinyl aromatic hydrocarbon copolymer is obtained by using a quantity of vinyl aromatic hydrocarbon not greater than 25% by weight to the total weight of the conjugated diolefin and of the same hydrocarbon.
As a consequence of this, the olefin fraction of the resulting copolymer preferably comprises a quantity of 1,2 structure ranging between 40% to 60% by weight to the total weight of the same copolymer.